1. Technical Field
The present invention relates to an optical amplification technology, and more particularly, to a technology of coupling signal light and pumping light in order to amplify the signal light, an optical pumping device having an efficient structure for coupling the signal light and the pumping light, an optical amplifier, a fiber laser using the optical pumping device, and a multi-core fiber for an optical pumping device which is a component of the optical pumping device.
The present invention contains subject matter related to Japanese Patent Application JP 2006-104056 filed in the Japanese Patent Office on Apr. 5, 2006, the entire contents of which are incorporated herein by reference.
2. Related Art
Conventionally, as a structure for splicing an optical fiber for transmitting signal light and a plurality of optical fibers for transmitting pumping light to one end (an input side) of an optical fiber for optical pumping, for example, devices disclosed in Patent Documents 1 and 2 were suggested.
FIG. 2 is a view showing an optical fiber device disclosed in Document 1 (Japanese Patent Publication No. 3415449). In an optical fiber device 1, a plurality of multi-mode fibers (hereinafter, referred to as MMFs) 2 for coupling pumping light are bundled up, a cross sectional area reduction section 5, in which the cross sectional area is reduced to the cross sectional region of a clad pumping fiber 4, is formed at the side of the output side of a fiber bundle 3, and the output side of the cross sectional area reduction section 5 and one end of the clad pumping fiber 4 are spliced in a splicing point 6.
FIGS. 3A and 3B are views showing an optical fiber device disclosed in Patent Document 2 (Japanese Patent Publication No. 3353755). In an amplification device 10A shown in FIG. 3A, the output sides of an optical fiber 11 for transmitting signal light and a output side of a plurality of optical fibers 12 for transmitting pumping light are inserted into and held on to a first ferrule 13, one end of an amplification optical fiber 14 is inserted into and held on to a second ferrule 15, and the output side face of the first ferrule 13 and the output side face of the second ferrule 15 are coupled with a graded index lens 16 interposed therebetween. In addition, in an amplification device 10B shown in FIG. 3B, the output side face of the first ferrule 13 and the output side face of the second ferrule 15 are abutting each other.
However, the above conventional technologies have the following problems. In the conventional technology disclosed in Patent Document 1, in a process of bundling up the plurality of MMFs to reduce the cross sectional area, a surface tension for filling gaps among the MMFs is exerted such that the cross-sectional shape of the MMFs positioned at an outermost circumference is apt to be deformed from circle cross-sectional shape. In addition, due to the deformation of the MMFs, coupling efficiency of pumping light with the clad pumping fiber deteriorates. In association with this problem, as the number of pumping ports increases, a deformation ratio of the cross-sectional shape increases and the coupling efficiency deteriorates. Accordingly, expandability is inferior and it is difficult to cope with the request for higher-power pumping.
The deformation of the MMFs occurs by the surface tension for filling the gaps as described above.
In general, when the fibers are only bundled up, a close-packed structure is formed (here, an example of bundling up one signal port fiber 48 located at the center thereof and six MMFs at the periphery thereof will be described), as shown in FIG. 5. The fibers are unified by flame fusion and the diameter thereof is reduced by elongation. However, at this time, since glass is soften, surface tension, that is, the force for filling the gaps is exerted. As a result, the cross-sectional shape of the unified portion is closer to a circle, compared with the original shape. As a result of this deformation, the shape of a light guide section of each MMF is distorted (FIG. 6 is a pattern diagram showing an example of the cross section).
In theory, when the diameter of the light guide section is reduced, a relationship between the diameter Din and the numerical aperture NAin of an input side and the diameter Dout and the numerical aperture NAout of an output side has a relationship expressed by Equation A.Din×NAin=Dout×NAout  Equation A
When the cross-sectional shape after elongation is distorted like this example, Dout of Equation A becomes a shortest diameter, that is, a shorter diameter, if the cross-sectional shape is an ellipse (example), and NAout of the emitted light increases.
As a result, when the numerical aperture NA of a rare earth-doped optical fiber connected to the output side of the device is fixed, the numerical aperture NA of the output side of the device exceeds the NA of connected rare earth-doped fiber and, as a result, splicing loss between the rare earth-doped optical fiber and the device significantly increases.
In consideration of the above-described deformation, the fiber bundle preferably needs to have a close-packed structure in order to reduce gaps between the fibers. In other words, the deformation is large except for a case where 7 or 19 fibers are bundled up as shown in FIG. 5 or 7, that is, the close-packed structure, hence there are practical problems. Even when 19 fibers are bundled up, which is close-packed structure, the deformation of the fiber is generally large. As a result, when the number of fibers is more than or equal to 19, a difficulty in use or manufacture usually occurs even when it is a close-packed structure.
Due to the deformation problem, the structure cannot be applied when at least 37 fibers are bundled up even in the close-packed structure and the number of ports is insufficient to obtain a large output power.
Even when only 10 pumping port fibers (MMFs) 49 are required, 18 pumping port fibers (MMFs) 49 and one signal port fiber 48 are required due to restriction of such close-packed structure. That causes redundant increase of Din of Equation A and, as a result, Dout or NAout is restricted. Therefore, problems may be caused in splicing with the rare earth-doped fiber at the output side.
In the conventional technology disclosed in Patent Document 2, since spatial propagation is included in the coupling portion between the lens system and the optical fiber, polish treatment and a film for preventing reflection need to be provided on the end of the optical fiber, both ends of the lens, and the end of the optical fiber for amplification. Accordingly, manufacturing cost increases. In addition, when contamination or dust exists on any of the end faces, the light is absorbed by the contamination or dust. As a result, if high-power light is inputted, a failure may occur due to heat generation by the absorption. Since the fiber bundle and the optical fiber for amplification are not directly spliced, long-term mechanical reliability is inferior and a failure is concerned. The mechanical failure of this air-path portion has significant influences on the characteristics of the system. In addition, since the mechanical failure must be necessarily avoided in view of safety, higher reliability is necessary.
The present invention is contrived to solve the above-described problems. An object of the present invention is to provide an optical pumping device for efficiently coupling signal light and pumping light to a double clad fiber for optical pumping.